1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a copier, a printer, or a facsimile machine.
2. Description of the Related Art
In general, an image forming apparatus that utilizes electrophotography has: a photosensitive member which serves as an image bearing member; a charging device (e.g., corona charger or charging roller) which charges a surface of the photosensitive member; an image exposure device for forming an electrostatic latent image on the photosensitive member; a developing device for developing the electrostatic latent image; a transfer device for transferring a toner image to a transfer material; a cleaning device which cleans residual toner off the photosensitive member; a residual charge eliminating exposure device for eliminating the electrostatic latent image on the photosensitive member; and a fixing device for fixing the toner image on the transfer material.
In the conventional image forming apparatus utilizing electrophotography, the photosensitive member which holds toner onto an electrostatic latent image generally has a photoconductive layer that includes a charge generation layer and a charge transport layer.
The photosensitive member moves by being driven in a given direction in response to a “start printing” signal.
The charging device applies a bias to the photosensitive member to charge the surface of the photosensitive member to a given electric potential (hereinafter referred to as a charging step).
The surface potential at this stage is called a VD potential. The surface of the photosensitive member is then irradiated with laser light or LED light which is controlled to be turned on/off based on a signal from a controller (hereinafter referred to as an exposure step). A spot on the photosensitive member that is irradiated with light is reduced in electric potential, and thus an electrostatic latent image is formed on the surface of the photosensitive member. The electric potential of a spot irradiated with light is called a VL potential.
Subsequently, a developing bias is applied to the developing device, which is placed to face the photosensitive member and which is filled with toner. This shifts the toner charged to a given level onto an electrostatic latent image on the photosensitive member, which is a photosensitive drum or the like, thereby turning the electrostatic latent image into a toner image (hereinafter referred to as a developing step). A developing bias is denoted by Vdev.
Thereafter, a bias having a polarity opposite to that of the toner on the photosensitive member is applied to the transfer member, which is a transferring roller placed adjacent to the photosensitive member and moving in the forward direction at approximately the same speed as the photosensitive member. In this state, the transfer material passes between the photosensitive member and the transfer member, with the result that the toner on the photosensitive member is transferred to the transfer material (hereinafter referred to as a transfer step).
The exposure step sometimes generates residual charges in the photosensitive member, causing VL to fluctuate during an image formation. VL fluctuates also due to a friction between the photosensitive member and components with which the photosensitive member is in contact, such as the charging member, the exposure member, and the cleaning member, and due to a rise in temperature that is caused by heat dissipated from the fixing device or other components while the photosensitive member is moving. In other words, the exposure and moving of the photosensitive member in the process of forming an image causes the fluctuation of the development contrast, which corresponds to the difference between Vdev and VL. The fluctuation leads to variations in how much toner the photosensitive member holds (toner bearing amount) and invites fluctuations in image density on the transfer material. The development contrast (i.e. the difference in potential between Vdev and VL) is denoted by Vcont.
An image forming apparatus has been proposed which stabilizes image density by detecting VL of the photosensitive member with a sensor and by controlling image forming conditions according to results of the detection (U.S. Pat. No. 6,339,441). A problem of this image forming apparatus is an increase in cost and apparatus size due to the installation of the sensor and a space for installing the sensor.
Another image forming apparatus reduces fluctuations in image density when forming the same image on multiple sheets by selecting an appropriate number of revolutions of the photosensitive member with the charge elimination step and the charging step prior to the formation of an electrostatic latent image in accordance with the temperature and humidity in the vicinity of the photosensitive member (Japanese Patent Application Laid-Open No. 2005-300745). However, increasing the number of revolutions of the photosensitive member before latent image formation is a problem because it slows down the printing speed and lowers the productivity of the image forming apparatus.
As a solution to the above-mentioned problem, an image forming apparatus has been proposed which predicts VL of the photosensitive member from the temperature around the photosensitive member, the photosensitive member rotation time, and the photosensitive member stop time (how long the photosensitive member remains still without rotating), and by executing process control based on the predicted VL (Japanese Patent Application Laid-Open No. 2002-258550).
A study conducted by the inventors of the present invention has discovered not only that image density is dependent on humidity, but also specifically that VL fluctuations in the process of image formation are dependent on the absolute humidity of the atmosphere and that VL fluctuations include a drop in absolute value of VL as well as a rise in absolute value of VL. Therefore, VL fluctuations cannot be predicted accurately with the conventional art proposed in Japanese Patent Application Laid-Open No. 2002-258550, where the absolute humidity of the atmosphere around the photosensitive member is not taken into consideration, nor is the possibility of both a rise in VL and a drop in VL happening with time as the photosensitive member rotation time “counts up” or increases. This conventional art is accordingly incapable of appropriate image formation control and of obtaining an image of uniform or stable density. Hereinafter, a phenomenon that acts to raise the absolute value of VL with time as the photosensitive member rotation time counts up is referred to as “VL UP” and a phenomenon that acts to lower the absolute value of VL with time as the photosensitive member rotation time counts up is referred to as “VL DOWN”.
FIG. 2 is a conceptual diagram of the surface potential of a photosensitive member. As illustrated in FIG. 2, the difference between Vdev and VL, “Vdev−VL”, corresponds to Vcont. A larger Vcont means more toner is available to be developed on the photosensitive member and an accordingly higher image density. VL UP is a phenomenon where VL shifts in a direction indicated by an arrow A of FIG. 2 (direction in which the absolute value of VL rises), thereby reducing Vcont and lowering the image density. VL DOWN, on the other hand, is a phenomenon where VL shifts in a direction indicated by an arrow B of FIG. 2 (direction in which the absolute value of VL falls), thereby increasing Vcont and raising the image density.
VL UP and VL DOWN will be described below in detail.
Phenomena relevant to VL UP will be described first. In an L/L environment (low temperature-low humidity environment), for example, an environment where the temperature and the humidity are 15° C./10% RH, a continuous image formation even if only for several sheets causes the VL UP due to the image formation as illustrated in FIG. 3A. A study conducted by the inventors of the present invention has confirmed that the rate of increase in VL per unit time in the VL UP phenomenon is greater in an environment where the absolute humidity is lower.
VL UP is influenced by how long the photosensitive member has been stopped before image formation. As the photosensitive member stop time before the image formation increases, the amount of increase in VL occurring over the elapse of time from the start of rotation of the photosensitive member becomes larger. For instance, VL rises to V1 as illustrated in FIG. 3A when the photosensitive member stop time is long whereas, when the photosensitive member stop time is short, VL rises only to V2, which is smaller than V1, as illustrated in FIG. 3B.
The inventors of the present invention believe that the main cause of the VL UP phenomenon is an increase in number of residual charges in the photoconductive layer due to the exposure of the photosensitive member during image formation. To elaborate, the inventors believe that the cause of VL UP in an environment where the absolute humidity is low is an increased resistance of one of layers in the photoconductive layer which inhibits smooth movement and injection of electric charges. Forming an image in an environment where the absolute humidity is low thus causes residual charges to accumulate in a high resistance layer and results in VL UP. One way to predict the amount of VL UP is to estimate a time for image formation on the basis of the photosensitive member rotation time.
Residual charges generated in the process of forming an image gradually leave from the photoconductive layer to the ground after the image formation is completed and stopped. As the image formation stop time is longer, residual charges generated in the previous image formation becomes less so that the photosensitive layer falls into a state in which residual charges are prone to accumulate in the subsequent image formation. Therefore, as an image formation stop time is longer, influence of VL UP becomes more conspicuous and an amount of increase in VL becomes larger in the subsequent image formation.
The VL DOWN phenomenon will be described below. When a continuous image formation is performed, VL drops with time as the photosensitive member rotation time counts up as illustrated in FIG. 3C.
VL lowered by VL DOWN exhibits a tendency to return to a level closer to the original VL level when there has been a period of time without any image formation after a time of image formation, namely, the photosensitive member stop time, is longer. For instance, VL DOWN due to the precedent image formation lowers VL in the precedent image formation to V4 as illustrated in FIG. 3C. This VL DOWN occurs during the photosensitive member rotation time, i.e. during the precedent image formation. The initial VL in the subsequent image formation takes a value closer to V3, which is the original VL level, as the photosensitive member stop time is longer as illustrated in FIG. 3D.
The inventors of the present invention believe that the main cause of VL DOWN is a decrease in number of residual charges in the photoconductive layer. To elaborate, forming an image raises the temperature of the photosensitive member, thereby lowering the resistance of the photoconductive layer, and thus the inventors of the present invention believe that the cause of VL DOWN is the lowered photoconductive layer resistance which allows residual charges trapped in the photoconductive layer to exit the photosensitive member. VL DOWN thus takes place when the temperature of the photosensitive member rises with time as the photosensitive member rotation time counts up, which lowers the resistance of the photoconductive layer and reduces trapped residual charges. Factors that raise the temperature of the photosensitive member with time as the photosensitive member rotation time counts up are friction with members that are in contact with the photosensitive member, such as the developing member, the charging member, and the cleaning member, and heat dissipation from the fixing device and other components.
Depending on the temperature and humidity of an atmospheric environment where the image forming apparatus is set, one or both of VL UP and VL DOWN take place. In one environment, VL rises once and then drops as illustrated in FIG. 3E. In a different environment, VL falls once and then rises as illustrated in FIG. 3F.
As described above, VL fluctuations have absolute humidity-related factors in addition to temperature-related factors such as the temperature of an environment in which the image forming apparatus is set, the temperature inside the image forming apparatus, or the temperature around or of the photosensitive member itself. Appropriate image formation control and an image of uniform density therefore cannot be obtained with the conventional art proposed in Japanese Patent Application Laid-Open No. 2002-258550 which does not include predicting VL fluctuations.
Also, in the conventional art proposed in Japanese Patent Application Laid-Open No. 2002-258550, an image formation is controlled on the premise that only one of VL UP and VL DOWN takes place. Therefore, there is a problem in that, when both VL UP and VL DOWN occur during the production of a single image, an appropriate image formation control is not accomplished so that an image with uniform density cannot be obtained.
A study conducted by the inventors of the present invention has also revealed for the first time that VL fluctuations are dependent on the rotation speed of the photosensitive member. The VL DOWN amount of the photosensitive drum has been discovered to be smaller in print modes where the photosensitive member rotation speed is low, such as a thick paper print mode and a gloss paper print mode, than in a plain paper print mode where the transfer material is transported at a higher speed, even when the movement distances of the photosensitive member in the former and latter modes are the same as each other.
A reason for this is that friction with a member that is in contact with the photosensitive member, for example, the charging member, the exposure member, or the cleaning member, affects the respective member differently depending on the rotation speed of the photosensitive member, and hence the temperature of the photosensitive member rises more slowly at a lower photosensitive member rotation speed, when there is less energy transferred through friction.
Conventional art as the one proposed in Japanese Patent Application Laid-Open No. 2002-258550 only predicts VL fluctuations in a high-speed print mode such as a plain paper print mode, and does not consider the difference in VL potential fluctuations between the high-speed print mode and a low-speed print mode (e.g., thick paper print mode or gloss paper print mode). Conventional art therefore has a problem in that an image of uniform density cannot be obtained when printing is executed in one of the low-speed print modes.